Sunday, July 26, 2015

For the fossil enthusiast in Minnesota, there are few groups with a bigger gap between "what they show in the books" and "what you actually find" than crinoids. The ideal of a crinoid fossil is a long segmented stem or stalk connected to a cup-like structure crowned by a group of delicate feathery arms. The Digital Atlas of Ordovician Life and the UGA Stratigraphy Lab's pages on Cincinnatian fossils have many images of these ideal crinoid fossils, if you're feeling like living vicariously. The reality in Minnesota is a bit more like this:

Saturday, July 4, 2015

One of the most diverse fossil groups in the Paleozoic rocks of Minnesota is also one of the least familiar to the layperson. This group is the Ostracoda, or "seed shrimp", a class of crustaceans that is still thriving. They owe their lack of familiarity to their diminutive size; they aren't called seed shrimp for nothing. Although found practically anywhere you can rub a couple of water molecules together, they are often more or less invisible to humans because most of them are on the order of 1 mm long. They owe their abundance in the fossil record to their durable shells, being equipped with a pair of valves not unlike a clam. Inside the valves is the tiny crustacean itself, with several pairs of appendages and distinct internal organs. With around 8,000 living species, there's a lot of diversity to go around. Ostracodes (also spelled ostracods) inhabit marine, freshwater, and moist terrestrial settings, are free-swimming or bottom dwellers, and include predators, herbivores, and detritivores. Definite ostracodes appeared during the Ordovician; there are some suspiciously ostracode-like creatures in the Cambrian, such as bradoriids, but they are not currently considered ostracodes. Fossil ostracodes are of particular interest for paleoenvironmental studies (depth, temperature, salinity, etc.) and biostratigraphy. There is one slight drawback to appreciating them as fossils, though: their size. Without a good microscope and the equipment to extract ostracodes, their charms will necessarily be vicarious for most people. Nothing against microfossils (I personally have a fondness for forams, because one of my first projects involved them), but those are just the practical breaks. There is, however, one exception in the rocks of the Twin Cities, a giant among ostracodes, the form commonly known as Eoleperditia fabulites or Leperditia fabulites. This species tops out around a centimeter long and resembles a bean. It can be both common and recognizable in the Platteville Formation.

This is the fabled giant ostracode Eoleperditia (or Leperditia) fabulites, from the University of Minnesota collections.

And here are examples in the wild, in the Platteville Formation (probably Mifflin Member).

Stauffer and Thiel (1941) have their usual list, featuring several dozen species, but it probably won't do much for you unless you have the requisite facilities or are just really good at spotting sesame-seed-sized fossils. Also, there are many publications specifically dedicated to the ostracodes of the Platteville and Decorah, and they're going to be more useful than an uncited faunal list. If you want the true Minnesota Ordovician ostracode experience, a selection of citations includes Ulrich (1890, 1892, 1897), Kay (1934, 1940), Hansen (1951), Cornell (1956), Swain et al. (1961), Swain (1987), Swain and Cornell (1987), and Johnson et al. (1991).